This invention relates the formation of an image or pattern in an article such as a metal coated substrate. More specifically, it relates to the formation of such an image or pattern using a high energy source such as a laser or flash lamp.
Surface layer materials are often imaged or patterned for many utilitarian purposes. The surface layers may include vacuum deposited thin films, solution coatings, and electroless or electroplated films. Patterned conductive surface layers may find use in both passive and active electronic circuits, display components, antennas for radio frequency identification tags (RFID), wireless local area networks (LAN), and proximity detectors as well as antennas for communication such as pagers, cell phones, and satellite reception. Optical surface layers may find application as optical components such as diffractive optical elements and security images, or in telecommunication applications as components that can perform optical switching, modulation, and multiplexing or demultiplexing.
There are techniques for forming patterns in surface layers. Two useful methods are chemical etching and laser ablation. Images or patterns created by chemical etching are formed by selectively dissolving the surface layer with the appropriate chemicals or by energy ablation are formed by explosively detaching and removing a thin surface layer in a selective manner to create an image or pattern. However, each of these methods has limitations.
Chemical etching is a multiple step process that may create hazardous waste. Typically a chemical or photo-resist is applied to selected parts of a surface that is to be patterned or imaged. Then, a chemical is applied on the entire surface and is able to remove the coating (e.g. copper) exposed on the surface but not the coating on the part of surface covered by the chemical or photo-resist. The chemical solution containing the dissolved surface layer material is then washed off of the imaged article. The often hazardous solution is collected and treated in a safe manner at some expense. This method is undesirable due to the multiple process steps and the imaged article can have residual photo-resist residue and undercut sidewalls of the image.
Contemporary methods of utilizing lasers to pattern or micro-machine materials rely on the physical phenomena of ablation. Energy ablation is a simpler process that does not involve hazardous waste disposal. Typically, the surface layer of an article to be imaged is exposed (for example through a mask) with light pulses from a high-energy source, such as a laser or a flash lamp. These pulses of energy are absorbed by parts of the surface layer not covered by the mask, and the energy impacting the layer causes a sudden increase in surface temperature for a short time. The rapid rise in temperature causes the surface layer material to explosively detach or eject from the substrate and create a pattern corresponding to the mask pattern.
An improved method for imaging or patterning surface layer materials has been created which comprises the steps of:
A. providing a substrate having a coating and an interface comprising the thin region where the coating and the substrate are closest to each other;
B. exposing at least one part of the total area of the coating to a flux of electromagnetic energy sufficient to disrupt the interface but insufficient to ablate the coating; and
C. removing the parts of the coating in registry with the portion of the interface area that was disrupted.
The substrate can be any material suitable as a support for a radiation-induced image. This process is particularly advantageous for substrates that are susceptible to heat damage such as some polymeric materials including polyester, polycarbonate, polyvinylchloride, and polyimide. The coating can be any imageable material that absorbs radiation. Typical coatings include metals, metal alloys, and inorganic compounds such as metal oxides and metal nitrides as well as organics. The flux of electromagnetic energy may be from a light source such as a laser or a short pulse width flash lamp.
The energy fluence is below the ablation threshold of the surface coating but sufficient to disturb the interface between the coating the substrate. Ablation threshold is the minimum energy needed to ablate a coating from a substrate and is dependent on the coating, substrate and wavelength of energy used. For purposes of this description, the term disrupt, as applied to the interface, means to affect the interfacial bonding between the substrate and the coating so that this interface is weaker than the bonding in the interface regions that have not been exposed to the energy flux. This weakening of the interfacial bond is sufficient to allow the removal of the coating in regions exposed to the energy flux as described herein without removing coating from the unexposed regions.
Less fluence, i.e., energy density at the coating surface, is required to disturb or disrupt the interface than is required in ablation processes, which implies greater throughput or output for a given Source of electromagnetic energy flux. Also, there is essentially no redeposition of coating material onto the work piece, which alleviates any detrimental effects of the imaged substrate associated with debris in the article produced.
A reflective, absorptive or diffractive mask defines the desired pattern. As an example, opaque reflective regions and transparent regions define the reflective mask patterns. When a uniform energy flux is incident upon the mask, the energy is reflected by the reflective regions and transmitted by the transparent regions resulting in the exposure of the desired parts of the coating material (corresponding to the pattern) to the energy flux.
The coating that is over the disturbed part of the interface is removed by a method such as contacting it with an adhesive roll, exposing it to high velocity stream of a benign liquid or gas (eg. air or water jet), or ultrasonic agitation in an aqueous solution. As used in this paragraph, the term benign means characterized by having no damaging effect (eg. by chemical reaction, corrosion, or physical erosion) on the coating or substrate. This step of removal of the coating over the disrupted area is relatively inexpensive.
Imaged articles are also part of this invention. Inventive articles comprising an imaged coating on a substrate are differentiated from articles made by ablation by a substantial absence of heat distortion and debris comprising the coating material. They are also differentiated from articles made by chemical or photo-resist process by a substantial absence of photo-resist and absence of undercut of the image which can occur with the chemical patterning process.
Applications that would benefit from this invention include patterning of inorganic thin films for active and passive electronic circuits, antennas for RFID tags, EMI shielding, patch antenna, and biosensinig pattern arrays. Patterned optical surface layers made by this invention could also find use in optical wave guides, electro-optic filters and modulators, holograms, security images, graphics and retroreflective materials. Patterned transparent conductors on both rigid and flexible substrates would find application in liquid crystal display (LCD) computer displays, televisions, touch screens, heated and electrochromic windows.